* Spell damage shard can be imbued on item if it doesn't already have a magical stat from 'stat shard' category. You can always imbue them on ear, finger, amulet, clothes and miscellaneous slot items even if they would have other magic as long as they don't have stat from 'spell damage' category.

* Damage type shard can always be imbued on weapon that do 100% phys damage regardless of other magics on the item.

* Resistance shard can be imbued on item if it doesn't already have a magical stat from 'stat shard' category. You can always imbue them on torso and head slot items even if they would have other magics as long as they don't already have a resistance stat.

* Skill shards may always be imbued on any item that already doesn't have any skill magic on it.

* General shards may always be imbued on items regardless of other stats.

* Special shards will follow their own rules based on shard types. For example eternal magic can be imbued on any magical item if it already doesn't have that magic imbued on it.

Upgrading stats with shards
It is possible to upgrade stats on magical items with shards. When you imbue a shard on the item it will replace the previous stat on it. For example you imbue +2 strength shard on and item that has +1 strength on it so the +1 gets replaced by +2. There are some limitations to this though. You may not upgrade a negative stat into positive and you must always imbue a more powerful shard on the item to replace the stat, meaning you cannot downgrade stats. Another limitation is that if the item has multiple stats of same type you may not upgrade any of those stats. For example if an item has +1 to strength and +1 to dexterity you cannot upgrade any stats from 'stat shard' category. Generic and damage type shards are excluded from this rule.